Blocks Programming Topics

Introduction

Important: This is a preliminary document for an API or technology in development. Apple is supplying this information to help you plan for the adoption of the technologies and programming interfaces described herein for use on Apple-branded products. This information is subject to change, and software implemented according to this document should be tested with final operating system software and final documentation. Newer versions of this document may be provided with future betas of the API or technology.

Block objects are a C-level syntactic and runtime feature. They are similar to standard C functions, but in addition to executable code they may also contain variable bindings to automatic (stack) or managed (heap) memory. A block can therefore maintain a set of state (data) that it can use to impact behavior when executed.

You can use blocks to compose function expressions that can be passed to API, optionally stored, and used by multiple threads. Blocks are particularly useful as a callback because the block carries both the code to be executed on callback and the data needed during that execution.

Blocks are available in GCC and Clang as shipped with the OS X v10.6 Xcode developer tools. You can use blocks with OS X v10.6 and later, and iOS 4.0 and later. The blocks runtime is open source and can be found in LLVM’s compiler-rt subproject repository. Blocks have also been presented to the C standards working group as N1370: Apple’s Extensions to C. As Objective-C and C++ are both derived from C, blocks are designed to work with all three languages (as well as Objective-C++). The syntax reflects this goal.

You should read this document to learn what block objects are and how you can use them from C, C++, or Objective-C.